This invention relates to a method of preventing over-discharge of a battery, and to a battery pack which uses that method. In particular, this invention relates to an over-discharge prevention method and battery pack optimized for lithium ion rechargeable batteries.
Turning to FIG. 1, a circuit diagram of a battery pack which contains a battery over-discharge prevention circuit is shown. The battery pack in this figure has an over-discharge protection switch 3 connected in series with batteries 1. When battery voltage drops below a minimum set voltage, the over-discharge protection switch 3 is turned on to prevent over-discharge of batteries 1. The over-discharge protection switch 3 is switched on and off by the control circuit 4. The control circuit 4 detects battery voltage and switches the over-discharge protection switch 3 off when battery voltage drops below the minimum set voltage.
A field effect transistor (FET) is used as the over-discharge protection switch 3. This is because FET internal resistance in the on state is small, and voltage drop and power loss can be minimized. The FET has a parallel connected parasitic diode. The parasitic diode maintains the FET in an on state with respect to current flowing in the reverse direction. As a result for example, even if battery voltage drops below the minimum set voltage and the FET over-discharge protection switch 3 is turned off, it is still possible to charge the batteries 1.
The battery pack protection circuit shown in FIG. 1 operates in the following manner to prevent over-discharge of batteries 1.
(1) When battery voltage is greater than the minimum set voltage, the control circuit 4 holds the over-discharge protection switch 3 in the on state. PA1 (2) When the batteries 1 are discharged and battery voltage drops below the minimum set voltage, the control circuit 4 turns the over-discharge protection switch 3 off to prevent over-discharge of the batteries 1. When the over-discharge protection switch 3 is turned off, current cannot flow in a direction which discharges the batteries 1 and discharge is stopped.
When battery voltage drops below the minimum set voltage in a battery pack which operates as described above, the over-discharge protection switch is turned off. Since load current cannot flow once the over-discharge protection switch is turned off, battery voltage rises. When battery voltage rises above the minimum set voltage, the over-discharge protection switch turns on allowing load current to flow, and battery voltage once again drops. When battery voltage rises and falls in this manner, the over-discharge protection switch is turned on and off and the batteries are repeatedly connected to and cut-off from the load.
To avoid this problem, a battery pack having hysteresis between the minimum set voltage for turning the over-discharge protection switch off and the reset voltage for restoring the over-discharge protection switch to the on state was developed and cited in Japanese Non-examined Patent Publication No. 4-33271 issued Feb. 4, 1992. Specifically, battery pack reset voltage was set higher than the minimum set voltage. The hysteresis voltage or the difference between the reset voltage and the minimum set voltage was, for example, from 0.5V to 5V per cell for the battery pack of this disclosure. When the hysteresis voltage was too small, the over-discharge protection switch would switch on and off causing switch chattering. To avoid this problem, hysteresis voltage was made larger. Large hysteresis voltage resulted in failure of the over-discharge protection switch to turn on during battery charging which made efficient battery charging impossible. Consequently, the battery pack cited in this disclosure had the drawback that hysteresis voltage was difficult to set.
The present invention was developed to overcome this drawback. Thus it is a primary object of the present invention to provide a method of preventing battery over-discharge and a battery pack which uses that method that can avoid over-discharge protection switch chattering with an extremely simple method and circuit.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.